Oxygen is required in increasingly greater quantities in industrial combustion and oxidation processes. Industrial processes for separating air into oxygen and its related constituents are typically based upon cryogenic distillation, adsorptive separation, chemical absorption and differential permeation through membrane media. Many analysts believe that the high equipment and operating costs of these processes have stifled the use of oxygen-based technologies in certain fields. Therefore, considerable effort is being undertaken to find more economical methods for producing oxygen. One of the most promising new technologies involves inorganic membranes formed from multicomponent metallic oxides such as titania-doped yttria-stabilized zirconia (YSZ) and praseodymia-modified zirconia.
Processes for separating air which utilize inorganic membranes formed from multicomponent metallic oxides are typically operated at high tempratures (e.g. 800.degree. C. or more) wherein the membranes conduct both oxygen ions and electrons. When a difference in oxygen partial pressure exists on opposite sides of the inorganic membrane and operating conditions are properly controlled, pure oxygen is produced as oxygen ions migrate to the low pressure side of the membrane while an electron flux occurs in the opposite direction in order to conserve charge.
Inorganic membranes formed by depositing thin films of stabilized zirconia onto porous substrates have been prepared by numerous methods including chemical vapor deposition (CVD) and electrochemical vapor deposition (EVD). CVD processes utilize a plurality of gaseous precursors which react by an activated process to form one or more solid products which are deposited onto the surface of a substrate. Typical CVD processes comprise (1) transporting gas phase reactants to the substrate and (2) reacting the precursors to form a crystal growth of the solid on the substrate.
EVD is a specialized chemical vapor deposition technique which employs ionic conductivity as well as electronic conductivity of the mixed conducting oxide to deposit the desired layer of oxide onto the surface of the substrate. The process involves contacting a mixture of metal halides on one side of a porous substrate and an oxygen source on the other wherein the reactants inter-diffuse into the pores of the porous substrate and react to form a multicomponent metallic oxide which is deposited on the pore walls. Continued deposition causes pore narrowing until eventually the pores become plugged with the multicomponent metallic oxide.
Continued growth of the deposit can then occur due to conduction of oxygen from the oxygen source through the deposited plug of multicomponent oxide.
Pal and coworkers at Westinghouse R & D Center, Pittsburgh, PA (High Temperature Science 27 (1990) 251) have disclosed an EVD process wherein a multicomponent metallic oxide is deposited in the pores of a porous substrate by contacting various metal halides corresponding to the metals in the multicomponent oxide and a mixture of hydrogen and water on opposite sides of a substrate. The reactants diffuse into the substrate pores and react to form the multicomponent metal oxide which is deposited onto the walls of the pores. Following closure of the pores, film growth may continue on the surface of the porous substrate by EVD.
A. J. Burggraaf and coworkers (J. European Cer. Soc., 8 (1991) 59) have disclosed a method for depositing yttria-stabilized zirconia films onto porous substrates having relatively large pores ranging in diameter from 0.2 .mu.m to 0.4 .mu.m. An initial CVD deposition is believed to cause pore narrowing although not confirmed by measurements of gas permeation as a function of pressure drop. In a subsequent EVD stage, a film of yttria-stabilized zirconia having a thickness of 1.5 to 8 .mu.m was deposited on the surface of the substrate by reacting YCl.sub.3 and ZrCl.sub.4 with oxygen which is electrochemically conducted by the deposited layer at temperatures above 1000.degree. C. X-ray diffraction experiments indicated that the material had a cubic structure, orientated in the [100] direction with a minor monoclinic or tetragonal phase.
While the above-mentioned methods are capable of producing thin gas-tight stabilized zirconia films on porous substrates, a need in the art exists for an improved process for depositing an ultrathin dense layer of less than about 0.5 .mu.m of a multicomponent metallic oxide onto a porous support in order to achieve enhanced oxygen flux of the membrane. Desirably, processing temperatures would be reduced compared to prior art processes in order to promote formation of low stress coatings, to reduce chemical reactions between the dense multicomponent oxide layer and the porous substrate and to minimize changes in the substrate microstructure typically caused by heating the material at unduly high temperatures.